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我为我的测试项目启用了可空性上下文并尝试修复所有可空性警告。除了以下我不理解的（return ref book;在线）之外，我能够修复它们。我在编译器生成的这一行的注释中粘贴了警告：

class Program
{
    private Book[] books = { new Book("Call of the Wild, The", "Jack London"),
                    new Book("Tale of Two Cities, A", "Charles Dickens") };
    private Book? nobook = null;

    public ref Book? GetBookByTitle(string title)
    {
        for (int ctr = 0; ctr < books.Length; ctr++)
        {
            ref Book book = ref books[ctr];
            if (title == book.Title)
                return ref book; //CS8619: Nullability of reference types in value of type 'Book' doesn't match target type 'Book?'.
        }
        return ref nobook;
    }
}

public class Book
{
    public readonly string Title;
    public readonly string Author;

    public Book(string title, string author)
    {
        Title = title;
        Author = author;
    }
}

ref Book book我不明白为什么编译器对方法中返回的可空变量不满意ref Book?

据我所知，我们可以将不可为空的变量分配给可空的变量，如下所示。正如下面的代码所示，如​​果我在具有类型的方法中返回非引用Book book变量，编译器不会发现任何问题：Book?

    public Book? GetBookCopyByTitle(string title)
    {
        for (int ctr = 0; ctr < books.Length; ctr++)
        {
            ref Book book = ref books[ctr];
            if (title == book.Title)
                return book; //no warning here. The compiler is satisfied if we don't use ref return value
        }
        return null;
    }

为什么编译器会在第一个代码片段中产生此错误：

Nullability of reference types in value of type 'Book' doesn't match target type 'Book?'.

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1 回答 1

4

发生错误是因为您在ref return此处使用 s。

回忆一下ref returns 的含义。您正在返回对变量的引用，调用者可以使用引用更改变量的值。

GetBookByTitle声明返回一个可为空的引用。根据声明，我可以从方法中获取引用，然后将其设置为 null：

ref var book = ref GetBookByTitle("Tale of Two Cities, A");
book = null; // book is nullable, right? GetBookByTitle says it will return a nullable reference

由于我传入的特定标题，book = null;将达到，将不可为空 books[1]设置为null！如果允许这样做，则会破坏可空引用类型带来的安全性，因此是不允许的。

于 2020-05-17T10:29:50.223 回答